In a contemporary detection system, a viewed target or scene forms a single image upon a focal-plane detector array including a large number of discrete detector elements that are highly responsive to electromagnetic energy within a pre-selected wavelength range. The electrical outputs of the detector elements are communicatively linked to sophisticated signal processing circuitry. By rapidly analyzing the pattern and sequence of detector element excitations, the processing circuitry can identify and monitor sources of electromagnetic radiation that appear within a scene or field of view.
When it is desired to view a scene over different portions (wavelength ranges) of the electromagnetic spectrum, the scene is filtered through one or more optical filtering elements. In a traditional system, mechanically movable filters are interposed into an optical path defined by a focusing element and a corresponding section of the detector array onto which that focusing element projects an image of the scene. Generally, such filtering elements are selectively situated intermediate the focusing element and the detector array and are incorporated in what is know in the field as a “filter wheel” that is rotated to alter the wavelength ranges over which a scene is observed.
Although filter wheels are still used in some applications, practitioners in the field of spectral imaging have recognized shortcomings of systems that rely on the selective mechanical interposition of filters within an optical path in order to image a scene over different wavelength ranges. Among the disadvantages associated with such systems are the facts that they are inherently expensive, heavy, large and fragile. More specifically, the use of mechanisms to effect movement of the filters adds costs and weight to the detection system. More significantly, such mechanisms are mechanically complex and require a high degree of precision to obtain the desired results. Thus, the reliability and durability of moveable filters, and their drive mechanisms, are of particular concern. This is especially true in space-based applications wherein it is extremely difficult or impossible to conduct “field” repair of such systems. Mechanical movement of the filters also introduces an observation dead time associated with (i) the generation of control signals to initiate the filter change, (ii) settle-down times that depend on the inertial characteristics of the mechanical components, (iii) and slow speeds that may be necessary in order to preserve optical alignment, avoid setting up vibration, and prevent damage to fragile optics. In some military systems requiring extremely rapid response times, any time loss associated with filter switching may be highly undesirable or even unacceptable. Moreover, and quite significantly, moveable filters (e.g., filter wheels) provide spectral data that is necessarily sequential in nature. More specifically, a scene is viewed through a first filter and data representative of the scene is registered at the detector array and stored in computer memory. Subsequently, a second filter is moved into position to filter the scene over another wavelength range, and the procedure is repeated over as many filtered wavelength ranges as the particular application calls for. Much more desirable is the acquisition of all spectral data through all filter elements simultaneously. This is particularly important when viewing rapidly changing events such as missile launches, muzzle flashes, or other ephemeral events.
In recognition of the aforementioned considerations, multi-image detector assemblies have been developed. Such an assembly does not require the use of moveable filters or other optical components in order to sense different portions of the electromagnetic spectrum or otherwise modify the incoming source signal. Representative of such an assembly is that disclosed in U.S. Pat. No. 5,479,015 issued in the names of Rudman et al. on Dec. 26, 1995 (hereinafter, the '015 patent). The '015 patent is drawn to a multi-image detector assembly including an array of detector elements (e.g., a focal plane array) wherein the detector array includes a plurality of imaging-registering sections. Corresponding to each image-registering section is a focusing member that focuses an image of a scene upon the image-registering array section. The plural images focused upon the various array sections are, at least in their spatial aspects, substantially identical. Each focusing member defines, in combination with its corresponding array section, an optical path. Disposed within, and dedicated to, each optical path is an optical element that modifies the image transmitted along that optical path. The “optical elements” are optical filters that facilitate registration of various images of a single scene simultaneously over disparate wavelength regions within the electromagnetic spectrum.
Whether a filter wheel system or the more modern simultaneous image acquisition system, such as that described in the '015, is employed, the goal of each of these multi-filter systems has traditionally been to provide independent measures of the same scene. That is, to “view” and analyze a single scene over multiple wavelength ranges in an effort to discern, in broad terms, the spectral content of activity or “events” within the scene. Some detector systems, however, are configured to detect the presence of emission over a single narrow range including a central wavelength of interest, for example. For instance, in certain applications, the presence of potassium in combusting material may be of particular interest. Combusting potassium emits electromagnetic energy having a wavelength of 766 nm. Accordingly, an illustrative traditional system singularly configured to detect the combustion of potassium would include a focusing element optically aligned with a detector-array to define an optical path and a single filtering element, configured to transmit a very narrow spectral band centered at 766 nm, interposed within the optical path. A shortcoming of such systems, however, is that the transmittance of an optical filtering element shifts or “drifts” with changes in temperature. That is, inherent to an optical filtering element is a temperature coefficient. Consequently, a system configured for the detection of burning potassium may function perfectly well when the filter is in an environment of between 25° C. and 30° C., but the limitations of the filtering element may render the system “blind” to the 766 nm central wavelength when the filter temperature is outside the illustrative 5-degree range.
Accordingly, there exists a need for a multi-filter detector assembly that facilitates detection of the presence within a scene of a predetermined central wavelength over a broader operational temperature range than has been heretofore realizable through the implementation of traditional, single-filter solutions.